Hypoxia abolishes the morning-night differences of metabolism and ventilation in 6-day-old rats.

Rat pups have a higher metabolic rate at night than in the morning; we questioned to what extent this was accompanied by changes in ventilation (Ve) and the effect of hypoxia on the morning-night differences. From birth, the pups were mother reared under a 12 h light: 12 h dark cycle (light on from 07:00 to 19:00). At day 6, oxygen consumption (VO2, by a flow-through method) and Ve (by airflow plethysmography) were measured at 07:30 and 19:30, and again at 07:30 of the next day. In normoxia, all values were higher at 19:30 than at 07:30. At all hours, Ve/VO2 remained constant (36.5 at 33 degrees C, 32 at 29 degrees C), because changes in Ve matched those of VO2. In acute hypoxia (inspired O2 = 10%), VO2 and Ve dropped significantly from normoxia, and more so at 19:30 than at 07:30, resulting in no difference in VO2, Ve, or Ve/VO2 among hours. These results suggest that in the rat pup Ve control is sufficiently developed to finely track changes in metabolic rate, and that hypoxia interferes with the metabolic and ventilatory morning-night differences.